The present invention relates to ferromagnetic thin film memories and more particularly to the fabrication of a thin film magnetoresistive memory device. The process for manufacturing the magnetoresistive bit lines involves depositing one or more layers of magnetic material over an underlying layer or surface, and etching the deposited material to a desired configuration.
U.S. Pat. No. 4,731,757 dated Mar. 15, 1988, U.S. Pat. No. 4,780,848 dated Oct. 25, 1988, U.S. Pat. No. 5,060,193 dated Oct. 22, 1991, and U.S. Pat. No. 5,496,759 which are assigned to Honeywell Inc. include background material on magnetoresistive memories and are hereby incorporated by reference.
The process for forming a magnetoresistive memory device provides first and second layers of thin film ferromagnetic materials separated by a layer of non ferromagnetic material and formed into an elongated shape having tapered ends to provide a magnetic storage element or a magnetic bit. In use, information is kept as a binary bit having one of two alternative logic levels stored in the two layers or films by having the magnetization point in one direction or the other(but opposite in each of these layers to the direction in the other), generally along the easy axis of magnetization. If the direction of magnetization is caused to rotate from such a direction by external magnetic fields, the electrical resistance of the layers will change with this magnetization direction rotation because of the magnetoresistive properties of the films. Measurement of the resistance allows the state of the memory to be determined. It has been observed that some number of bits will fail after a number of repeated memory storage and retrieval operations, or read/write cycles. The failure mechanism is believed to be bit edge reversal which is sometimes called edge spin reversal. The elongated shape of the bit structure causes large demagnetization fields along the edge of the layers. In such large demagnetization fields, electron spins at the edge of the films are constrained to lie nearly parallel to these edges and the direction of elongation of these films. During read or write operations it is intended that bit edge reversal does not occur.
In the past, proposed methods to prevent bit edge reversal have included using a magnetic spacer at the bit edge, applying an external magnetic field, using antiferromagnetic film on the edge of the bit and magnetic hardening of the bit edge.
The geometric configuration of a storage element has a great influence on its ability to provide failure free operation with wide operating margins. Wide operating margins are necessary for the production of high reliability memory devices.
Thus, a need exists for an improved understanding of storage element geometric configurations which will provide a magnetoresistive memory bit which allows an unlimited number of read/write cycles without bit edge reversal.